|Publication number||US6853446 B1|
|Application number||US 09/641,137|
|Publication date||Feb 8, 2005|
|Filing date||Aug 16, 2000|
|Priority date||Aug 16, 1999|
|Publication number||09641137, 641137, US 6853446 B1, US 6853446B1, US-B1-6853446, US6853446 B1, US6853446B1|
|Inventors||Gilad Almogy, Hadar Mazaki, Zvi Howard Phillip, Silviu Reinhorn, Boris Goldberg, Daniel I. Some|
|Original Assignee||Applied Materials, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (23), Non-Patent Citations (1), Referenced by (38), Classifications (13), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application claims priority from U.S. Provisional Patent Application No. 60/149,215, filed Aug. 16, 1999, and is incorporated herein by reference.
Wafer inspection systems are well known in the art. One conventional system, embodied in U.S. Pat. No. 5,699,447 uses normal illumination and bright-field detection (i.e., the illumination approaches the wafer at ninety degrees thereto). Another type of conventional system, as embodied in U.S. Pat. No. 5,825,482 uses oblique illumination and dark-field detection (i.e., the illumination approaches the wafer obliquely). A third type of system, as embodied in U.S. Pat. No. 5,982,921, uses normal illumination and dark field detection. All of these conventional approaches have advantages and disadvantages, some of which relate to the particular application or situation in which the system is used.
Under normal illumination, the surface of the object viewed is normal to the optic axis of the objective lens and light is used to illuminate the object. In a bright field system, light reflected back to the objective lens in a direction substantially parallel to the incident beam is used to form an image. Hence, surfaces that are reflective and perpendicular to the light rays appear bright and features that are nonreflective or oblique reflect less light back to the objective lens and appear darker. A dark field system may be implemented with either normal or oblique illumination. In either case, light that is scattered away from the optical axis is collected by dark field detectors positioned at an angle to the surface being viewed to form an image. Inclined surfaces of features such as ridges, pits, scratches, and particles therefore appear bright, providing enhanced contrast of these features from subtle topographic features. Thus, reflective features that normally appear bright in bright field illumination are completely black in darkfield illumination and subtle features that are undetectable using bright field illumination may be readily observed with dark field illumination.
In a laser-scan wafer inspection scenario it is sometimes preferable to illuminate the wafer at an angle normal to the wafer surface, while at other times preferable to use oblique illumination, depending on the details of the wafer materials, patterns and defects. The optical scattering characteristics of semiconductor wafers vary dramatically as the wafers proceed from one step to the next of the IC production flow. Some layers (such as bare silicon) are very smooth whereas some others (such as deposited aluminum) can be very rough and grainy.
It is well known that oblique illumination angles help reduce the unwanted optical scattering of the grains and roughness by the “Lloyd's mirror” effect (a destructive interference of the incident and reflected light at the surface which substantially reduced scatter from roughness and grains whose height from the surface is much less than the wavelength of the incident light, especially for metallic surfaces). Oblique illumination angles have, however, some limitations which make them less useful than normal illumination for some layers. One deficiency of oblique illumination angles is the inability of the light to penetrate between dense lines, such as those used in poly-silicon or metal interconnects. Another deficiency of oblique illumination is the dependence of the scattered signal on the direction of the substrate features (i.e., the loss of the symmetry which exists with normal illumination).
In practical inspection systems it is often desired to have replaceable optical elements which allow determination of the spot size. Such a system can thus be optimized for scanning with a large spot and obtaining a very high scan speed although a limited sensitivity; or, on the other hand, for scanning with a small spot and obtaining a very high sensitivity but at a lower scan speed. For normal illumination this is quite straightforward to do and only the classical resolution limits how small the spot can become. For oblique illumination, however, very small spots cannot be obtained due to the additional geometrical factor which introduces spot spread across the substrate plane which is inclined to it.
Accordingly, a need exists in the art for an improved wafer inspection system selectively and advantageously permitting use of either normal scanning illumination or oblique scanning illumination, based on the particular optical scattering characteristics of a semiconductor wafer at a time of inspection.
An advantage of the present invention is a wafer inspection system selectively and advantageously permitting use of normal scanning illumination or oblique scanning illumination to optimize the inspection characteristics of a scanned layer.
According to the present invention, the foregoing and other advantages are achieved in part by a variable illumination angle substrate inspection system. The variable illumination angle substrate inspection system comprises: a light source providing a light beam; a scanner imparting scanning deflection to the light beam to provide scanning beam approaching the substrate at a first angle; and a deflection element selectively insertable into optical path of the scanning beam and deflecting the scanning beam so as to approach the substrate at a second angle.
Another aspect of the present invention is a variable illumination angle inspection system for inspecting a substrate including a light source providing a light beam and a scanning element adapted to output the light beam along a first optical path to the substrate, the first optical path including a portion incident to the substrate and forming a first angle relative to the substrate. A deflection element is selectively introduced into the first optical path to output the light beam along a second optical path to the substrate, the second optical path including a portion incident to the substrate and forming a second angle relative to the substrate, wherein the first angle is different from the second angle.
In still another aspect, the present invention provides a deflection element for use in a variable illumination angle substrate inspection system. This deflection element includes a first deflecting surface and a second deflecting surface, wherein each of the first and second deflecting surfaces include a mirrored surface. The first deflecting surface is disposed at an angle with respect to said second deflecting surface so that an illumination beam entering the deflection element from a first direction is output from the deflection element in a second direction.
For the above reasons, and for reasons discussed herein, the present invention can therefore be optimized for the particular characteristics of a scanned layer.
Additional features and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein only preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.
A mechanism for scanning the laser beam is provided. This mechanism for scanning the laser beam may include, as well known in the art, a galvanometric scanning planar mirror, a rotating polygon mirror, an acousto-optic deflector (AOD), or any other mechanism for imparting the requisite laser scan motion to the laser beam, wherein this mechanism is represented in
In one basic configuration, the scanning element 120 includes a galvanometric scanning planar mirror rotated by a motor 126 able to adjust an angle of the mirror by fine predetermined increments in response to scanning instruction signals from a scanning controller 115, as known to those skilled in the art. In such an embodiment, the mechanism for scanning and the mechanism for deflecting are advantageously incorporated into a single element. The scan takes place around a plurality of central positions wherein each central position corresponds to a deflection of the scanned beam in preferred directions, such as along normal and oblique illumination channels. Two such illumination channels are shown in
Scanning element 120 deflects the light beam in a predetermined direction, such as toward the semiconductor wafer or substrate 105 or toward an optical device such as an objective lens 130 or mirror 140. The scanning can be performed, for example, along a first axis, such as the X-axis, while the wafer is moved by a scanning stage (not shown) along a second axis perpendicular to the first axis, such as the Y-axis. Other combinations of process variables such as the scanning speed, length of the scanning line, distance between adjacent lines, and light beam spot size can be employed to practice the present invention, as desired by the user.
In the position depicted in
To change the angle of illumination, actuator 190 introduces another mirror, mirror 170, to intercept the light beam output from the scanning mirror 120 and retracts the de-selected mirror 180, as shown in
The embodiments illustrated in
Thus, in accord with the above, oblique illumination may be obtained from a normal illumination scanning beam by introducing a different optical deflecting device, such as a prism or mirrored glass wedge, into a normal illumination scanning beam. This may be accomplished, for example, using actuator 195 and translatable actuator arm 185, or may be accomplished in any other manner of introducing an optical element into an optical path as can be appreciated by those skilled in the art.
For example, the optical deflecting element may include a partially mirrored glass wedge 500 disposed under objective lens 130, as shown in FIG. 5. Proper choice of the wedge geometry and glass index of refraction allows focusing of the oblique illumination at the same distance and position as illumination light provided in the normal direction along axis A coincident with a center of objective lens 130. In other words,
The original back focal length B is 15.8 mm and the objective back focal plane of objective lens 130 is positioned a distance D, 4 mm, from the top of the glass wedge. The distance T from the leftmost surface of the glass wedge to the chief ray, the ray passing through the center of the aperture stop of the optical system, from the objective is 8.08 mm. In the above configuration, the numerical aperture (NA) of the lens is selected to be 0.125. A range of NA between about 0.04 and 0.125 may be used, however, based on the selected parameters.
Although this aspect utilizes a right angle triangular prism, other shapes such as irregular polygonal shapes may also be utilized in accord with the invention. Additionally, the above defined dimensions embody only one specific example of a glass wedge providing oblique illumination and many other combinations of materials, angles, and dimensions may be employed to achieve the above described result in accord with the invention. In other words, in the example above, an apex angle of 30° is chosen to produce an angle of incidence of 60°, however, other incidence angles may be obtained by using different wedge 500 geometries and properties using the above principals and it is to be understood that the illustrative parameters above relate to a specific example and are in no way limiting to the inventive concepts disclosed herein. Further, although the glass wedge 500 is described as being positionable under objective lens 130, other variants may be advantageously be employed. For example, glass wedge 500 may be incorporated with objective lens 130 or may be embodied within a common structure so as to be simultaneously positionable within incident illumination light.
As shown in
In an automated inspection system, an autofocus mechanism is desirable. Some autofocus systems applicable to normal illumination configurations utilize the light reflected back through the objective. Such an autofocus mechanism may be accommodated by a modified unidirectional oblique illumination adapter. In the modified adapter, an autofocus prism 600 such as that depicted in
In this embodiment, the mirrored surface 520 on glass wedge 500 is replaced with a mirrored coating 620 transmitting a portion of the incident light. A bright specular reflection in the normal direction can then be at least partially transmitted back through the mirrored coating 620 to the objective lens 130 to permit bright field detector and autofocus operation. With reference to
Further, oblique illumination may be provided bidirectionally by replacing the glass wedge of
Additionally, the surface coatings 740 and 750 may comprise a plurality of layers adjacent one another, wherein the function of the coatings 740 and 750 are shared by the plurality of layers. For example, the s-polarizing beamsplitter coating 740 may utilize more than one layer, wherein the combination of layers produces a desired polarization level. Thus, a first s-polarizing beamsplitter coating 740 having a first polarization ratio may be used in conjunction with a second s-polarizing non-beamsplitter coating 741 (not shown) having a second polarization ratio provided just after the first s-polarizing beam splitter coating 740 to produce, in combination, the desired polarization level.
Alternately, the surface coatings 740 and 750 may be replaced by combinations of half-wave plates and polarizing beamsplitter coatings. For example, instead of s-polarizing beamsplitter coating 740, a p-polarizing beamsplitter may be disposed between two half-wave plates. The first half-wave plate would rotate the incoming polarization by 90 degrees, turning the s- into p- and vice versa, as known to those skilled in the art. The p-polarizing beamsplitter coating would transmit p- (originally s-) and the second half-wave plate would rotate the incoming polarization another 90 degrees, turning the incoming p-polarization into an output s-polarization. This embodiment additionally contemplates other combinations of wave plates, such as quarter wave-plates, and/or polarizing beamsplitter coatings to achieve a desired output polarization ratio. One polarization ratio suitable for use with the system of the invention is approximately 99:1.
The three-section adapter 790 is introduced into the light path, such as by actuator 190 and translatable actuator arm 185, to deflect light incident thereon toward a wafer or substrate at an oblique angle and focus the oblique illumination at the same distance and position as illumination light provided in the normal direction along axis A coincident with a center of the objective lens 130. In this aspect of the invention, the glass wedge is made of SFL6 glass, however, other variants of the three-section adapter 790 could utilize other conventional glasses, such as BK7. The glass wedge apex angle α is 30°, the width H at the upper surface is 10.88 mm, and the length L at the leftmost surface is 18.8 mm. End portion E may optionally be removed, as indicated in
The original back focal length B is 15.8 mm and the objective back focal plane of objective lens 130 is positioned a distance D, 4 mm, from the top of the three-section adapter 790. The distance T from the leftmost or rightmost surface of glass wedges 700 and 720, respectively, to the chief ray, the ray passing through the center of the aperture stop of the optical system, from objective 130 is 8.08 mm. The numerical aperture (NA) of the lens is 0.125. A range of NA between about 0.04 and 0.125 may be used, however, based on the selected parameters. It is to be understood that
The above defined dimensions embody only one specific example of glass wedges 700, 710, and 720, cooperatively used to provide oblique illumination and it is to be understood that many other combinations of materials, angles, and dimensions may be employed to achieve the above described result in accord with the invention and the illustrative parameters above relate to only one specific example and are in no way limiting to the inventive concepts disclosed herein. Additionally, three-section adapter 790 permit continued used of autofocus systems, as described above and as known to those skilled in the art, even when the primary illumination beam is oblique. Still further, as with the previous embodiments, one or more actuators may selectively insert different objectives into the optical path and may selectively insert three-section adapter 790 into the optical path, individually or in combination with an objective and/or focusing device.
In the configuration illustrated in
Further, because the two polarizations propagate in opposite directions, the scattered light has an azimuthal polarization dependence which can assist in defect discrimination. For example, the bidirectional illumination utilizing s- and p-polarized light, as described above, affords a small degree of height discrimination of defects, as shown in
Another embodiment of the present invention is shown in
As will be appreciated by those skilled in the art, the above systems provide a scanning beam arranged to scan an entire wafer in either a normal or an oblique mode. Moreover, with the assistance of scanning controller 115, the system may be adapted to provide selective normal or oblique scanning of individual portions of the wafer to, for example, enhance defect detection of those areas. In particular, the invention set forth in the appended claims permits detailed examination of specific coordinates of interest, as well as global examination of entire wafers from a preferred perspective.
Thus, the invention provides an apparatus for selectively and advantageously permitting use of normal scanning illumination or oblique scanning illumination to optimize the inspection characteristics of a scanned layer during wafer inspection. Various details are set forth herein to provide a thorough understanding of the present invention to those skilled in the art, although many explicit details of materials, equipment and methodology are not set forth herein in detail so as not to unnecessarily obscure the present invention. Only the preferred embodiment of the present invention and but a few examples of its versatility are shown and described in the present disclosure and it is to be understood that the present invention is capable of use in other combinations and environments and contemplates modifications within the scope of the inventive concept expressed herein.
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|International Classification||G01N21/95, G02B21/08, G02B26/10, G01N21/21|
|Cooperative Classification||G01N21/21, G02B26/10, G01N21/9501, G02B21/082|
|European Classification||G01N21/95A, G02B21/08B, G01N21/21, G02B26/10|
|Dec 7, 2000||AS||Assignment|
Owner name: APPLIED MATERIALS, INC., CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:ALMOGY, GILAD;MAZAKI, HADAR;PHILLIPP, TZVI HOWARD;AND OTHERS;REEL/FRAME:011380/0321;SIGNING DATES FROM 20001002 TO 20001029
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